We are more likely to feel sad if the Moon opposes our own Moon sign. So Aries Moons may be the trigger for you, or a square aspect, if the Moon is in Aries or Cancer. You can look up Moon tables online, try https://cafeastrology.com. The Moon moves through all twelve Zodiac signs each month. It spends about 2-2 1/2 days in each sign. Also, we all get more sad or emotional during the Full Moon.